He also attacked the inexperience of some of the agents assigned to the case, one of whom accepted she was "pretty green" and was learning "on the job".

Mr Owen was speaking on the final day of a hearing of the Special Immigration Appeals Commission (Siac) which will rule whether Zatuliveter should be deported.

Ms Zatuliveter, 26, met the Liberal Democrat MP for Portsmouth South in April 2006. The pair later began an affair. The Siac commission heard yesterday that she wrote in her diary like a "love-struck teenager", and made entries of a "highly personal nature".

She denies the Government's accusation that she is a member of Russian intelligence and was intentionally used to target and seduce Hancock, who sat on the Commons Defence Select Committee and chaired the All Party Parliamentary Group on Russia. She also denies that the diary entries are faked to lend her story credibility. Dates, travel details and information in the diary about her relationship look "entirely plausible", said the judge, Mr Justice Mitting.

The alleged spy was initially interviewed by MI5 in autumn 2009. In August 2010 she was detained at Gatwick Airport as she flew back from Croatia and interviewed by the Security Service.She was arrested in December last year and served with a deportation order.

Mr Owen said there was "not a jot" of evidence against her.

Siac hearings are unusual in that for large periods they are held in secret with neither Zatuliveter nor her lawyers allowed inside. Instead, a specially vetted adviser represents her but the official cannot relay details of the proceedings to her team. It is not known what evidence – if any – is being presented behind closed doors.